1. Field of the Invention
This invention relates to a color image reading apparatus and, more particularly, it relates to an apparatus adapted to form a color image of an original laid on an original supporting plane on at least three line sensors by means of an imaging optical system comprising an anamorphic lens and reading image data from the obtained color image. A color image reading apparatus according to the invention can find applications in image scanners, digital copying machines, facsimile machines and other apparatus.
2. Related Background Art
In a known flat-bed type image scanner as disclosed in Japanese Patent Application Laid-Open No. 3-113961, an imaging lens and a line sensor are rigidly held in position so that the surface of the original is exposed to light through a slit and image data are read from the original by driving only a plurality of reflecting mirrors in a scanning direction. In recent years, there have been proposed various carriage type image reading apparatus comprising a plurality of mirrors, imaging lenses and line sensors that are integrally combined to scan an original.
FIG. 1 of the accompanying drawings schematically illustrates a principal portion of a known carriage type image reading apparatus.
Referring to FIG. 1, the light beam emitted from a light source 701 is made to irradiate original 708 directly or by way of a reflecting hood 709 and the path of the light beam reflected from the original 708 placed on an original supporting glass panel 702 is folded within carriage 706 by means of first, second, third and fourth reflecting mirrors 703a, 703b, 703c and 703d before the light beam is focussed by an imaging lens 704 on a linear image sensor 705 such as a CCD (charge coupled device, to be referred to simply as CCD hereinafter). Image data are read from the original 708 by driving the carriage 706 in the direction of arrow A (sub-scanning direction) by means of a sub-scanning motor 707. In FIG. 1, the CCD 705 is formed by arranging a plurality of light receiving elements (pixels) in a one-dimensional direction (main-scanning direction).
The carriage 706 has to be down-sized if the image scanner having the above described configuration is to be down-sized. The carriage 706 may be down-sized by increasing the number of reflecting mirrors it contains and/or by using a single mirror to reflect the light beam for a number of times in order to secure a required length for the path of the light beam.
However, the above described techniques for down-sizing the carriage tend to make the internal structure of the carriage 706 highly complex and hence it is very difficult with any of such techniques to secure the level of precision required when assembling the apparatus. Then, the cost of precisely assembling the apparatus will inevitably be high. Additionally, the imaging performance of the apparatus will be degraded as a function of the planar precision of the reflecting mirror(s) and the number of times of reflection that occurs in the optical imaging system. It may be needless to say that a poor imaging performance adversely affects the image read by the apparatus.
On the other hand, the distance between the object to be imaged and the image formed by the imaging optical system can be reduced by using a wide angle lens for the imaging lens (imaging system) 704. As a matter of facts, various imaging systems that can be realized by using a practically feasible number of wide angle lenses having a spherical surface have hitherto been proposed. However, the half angle of view of such lenses is 25 degrees at most and the problem of field curvature and that of astigmatism can become remarkable to make it difficult for them to perform optically satisfactorily if a wider angle is used.
In view of these circumstances and as result of intensive research efforts, the applicant of the present patent application succeeded in dissolving the above problems by introducing an anamorphic lens having at least a surface that shows rotational asymmetry relative to the optical axis into the imaging system. This technique is disclosed in Japanese Patent Application Laid-Open No. 2000-171705 filed by the applicant of the present patent application.
On the other hand, there have also been proposed various color image reading apparatus adapted to form a color image of an original on at least three line sensors by means of an imaging optical system comprising an anamorphic lens and reading image data from the obtained color image. Since such a color image reading apparatus reads different sets of color data sequentially by means of at least three line sensors, there can arise occasions where any of the line sensors fails to read a same line on the original to consequently give rise to color separation in the obtained image. Particularly, if the imaging lens system is compactly configured, the obtained image can be distorted to give rise to discrepancies among the color data read by the different line sensors. While the discrepancies among the color data read by the different line sensors can be electrically corrected in the direction along which the pixels of each line sensor are arranged (main-scanning direction) by making use of the locations of the pixels, it is difficult to significantly correct the discrepancies in a direction perpendicular to the direction of arrangement of the pixels of the line sensors (sub-scanning direction) because of the volume of the data that have to be corrected.
Additionally, when an anamorphic lens is incorporated into an imaging optical system, the image forming performance of the imaging optical system is rotationally asymmetric relative to the optical axis of the system due to the influence of the anamorphic surface. Then, it is necessary to define the rotational phase of the anamorphic lens relative to the lense barrel.